Mobile transmit diversity beamforming refers to transmitting a signal in two streams over two respective transmit antennas, which may have the same or different amplitudes, with a transmit diversity parameter, such as a phase difference, applied to the first stream with respect to the second stream by a signal modifier (beamformer), connected to the transmit antennas ports. Due to fading and multipath effects, in order to produce effective mobile transmit beamforming, i.e., increasing the likelihood of constructive rather than destructive interference at the base station, a feedback signal from the base station is desirable to adjust the phase difference.
Power-control signaling is a technique used to minimize inter-channel interference and increase network capacity. For example, mobile communication standards include a high rate, continuous, power-control signaling to ensure that mobile communication devices do not transmit too much or too little power. More specifically, based on the strength of the signal sent from the communication device and received at the base station, the base station sends a power-control signal or power control bit to the mobile communication device indicating whether the communication device should increase or decrease the total power of its transmitted signal. The transmission rates for each value of the power-control signals are, for example, 1.25 ms for cdmaOne (IS-95)/CDMA2000, and 0.66 ms for WCDMA.
In some mobile transmit diversity systems, the feedback signal may be a power control signal. For example, in 3GPP R99, HSPA, and LTE Rel. 8, as well as CDMA2000 networks, the Mobile Unit (also referred to as user equipment, or UE) may have no direct knowledge of the channel impulse response of the different uplink transmit receive antenna pairs. Accordingly, a mobile transmit beamforming diversity system may use a quality-indication signal, which need not require any new or non-standardized dynamic feedback signaling between the network and the UE in HSPA. The base station may be unaware that the UE is in open loop beamforming transmit diversity mode, e.g., no changes may need to be made to the NodeB receiver processing (synchronization, channel estimation, demodulation, decoding) in order to accommodate UEs in this mode. In other mobile transmit diversity systems, the feedback signal may be a dedicated transmit diversity feedback signal, for example, instructing the UE to maintain or reverse a phase difference change, or providing a UE with a phase difference adjustment or value.
In order to correctly use information obtained from one or more feedback signals from the base station, it is desirable for the mobile device to correlate or otherwise match between a transmit signal or transmit diversity parameter, and the feedback signal that responded to that particular transmit signal or transmit diversity parameter. That is, the system delay between a mobile diversity transmission and a feedback should be determined by the mobile device.
One way to determine a system delay is based on a standard handshake already provided for, between the UE and the network upon initialization and upon other changes, e.g., base station handover. When identifying the type of network that serves it, the UE will also conclude from that info what is the correct delay. However, such identification is inadequate, as it may not always be done, or may not be done well, or may involuntarily be changed without notice.